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The Law of Superposition

The Law of Superposition. What is Superposition?. The Law of Super position states that in HORIZONTAL rock layers, the bottom layer is the oldest and the top layer is the youngest.

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The Law of Superposition

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  1. The Law of Superposition

  2. What is Superposition? • The Law of Super position states that in HORIZONTAL rock layers, the bottom layer is the oldest and the top layer is the youngest. • Think about making a layered cake: Which layer do you put down first? We start and the bottom and build to the top!

  3. How to rocks form layers? • Take a look at this video: • The Principle of Superposition

  4. Ages of Rocks • The relative age of a rock is its age compared to other rocks. Use words like: “older or younger” • The absolute age of a rock is the number of years since the rock was formed. Ex: 358-360 mya

  5. Relative Dating and Index Fossils • What is relative dating? • Any method of determining whether an event or object is older or younger than other events or objects. • What is an index fossil? • A fossil that is found in the rock layers of only one geologic age and is used to establish the age of the rock layers. • Is found in rock layers around the world, ex Trilobites

  6. Activity # 1 • On your desk, you have 8 large colored index cards with nonsense letters placed on them. • Your task is to determine what the correct sequence of the letters are. • You have two clues: • The card with the letters “C” and “T” is on the bottom, or the oldest layer • Look for a card that has either a “T” or “C” written on it for the second layer

  7. M D X O N B U A G C T This is one possible way to arrange the cards. Questions: What letter is the oldest? What letter is the youngest? What letter showed up the most? Which letters only showed up once? Which letters could be index fossils? How did you know which was older: “M” or “X”? MD DXO ON NB NBU UA AGC C T

  8. When does Superposition not apply? • When the layers are not horizontal, then Superposition does not apply. This can happen for several reasons. • We call these UNCONFORMITIES. Unconformities leave gaps in the rock record. • Let’s look at these examples: • Folded Layers • Faulted Layers • Formation of an Unconformity

  9. Plate Tectonics • If you notice, unconformities mostly are caused by the shifting of the Earth’s crust. Plates can move apart (diverge), together (converge), or slide past each other (transform or fault).

  10. Intrusion • An intrusion is when magma from the center of the Earth comes up and disrupts the layers of the rock that are already there. Take a look at this diagram: • Intrusion Unconformity The intrusion is the piece of rock that is coming upwards towards the surface. Can you determine the age of the layers from oldest to youngest?

  11. Other Clues to Relative Age • Clues From Igneous Rock • Lava that cools at the surface is called an extrusion. Rock below an extrusion is always older. • Magma that cools beneath the surface is called an intrusion. An intrusion is always younger than the rock layers around an beneath it.

  12. Using Fossils to Date Rocks! • Scientists use index fossils to match rock layers. • An index fossil must be widely distributed and represent a type of organism that existed only briefly. • They are useful because they tell the relative ages of the rock layers they are found in.

  13. The Trilobite • One example of an index fossil is a trilobite. • Trilobites were a group of hard-shelled animals whose bodies had three distinct parts. • They evolved in shallow seas more than 500 million years ago.

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